Diesel-electric railroad locomotives with series-wound, direct current (DC) traction motors replaced steam powered locomotives in the 1940's and have been the conventional motive power in America ever since. These railroad locomotives typically have either four or six axles with each axle having one traction motor and two wheels.
Such a railroad locomotive is set into motion by applying power to each of the series-wound, DC traction motors. Upon the application of power, torque develops in each of the series wound DC traction motors. This torque is then applied to the wheels of the axle. When enough torque has been developed, the wheels of the railroad locomotive are forced to turn, thereby moving the locomotive. The series wound DC traction motors are able to develop torque throughout their full speed range from stall to full speed.
A problem that can occur during acceleration of a railroad locomotive is wheel-slippage. Wheel-slippage occurs when the amount of adhesion existing between the wheels of one axle of the railroad locomotive and the rail of the railroad track is insufficient for the amount of torque being applied to the wheels. Thus, the wheels begin to spin on the rail. When wheel-slippage occurs, the traction motors continue to develop torque. This results in increasing the amount of wheel-slippage. Ultimately, the traction between the wheels of the railroad locomotive and the rails of the railroad are greatly diminished. To correct this runaway condition, the wheel speed must be reduced to nearly the rack speed of the locomotive (or the speed that the wheels should be turning to maintain the current speed of the locomotive). A problem that exists with railroad locomotives is reducing the speed of the traction motor associated with the wheel-slippage without affecting the speed or power of the other traction motors. Thus, in current systems, a degradation in the power being applied to the locomotive is realized when remedying wheel-slippage.
Several techniques have been used in an attempt to control wheel-slippage on railroad locomotives. A common technique involves reducing the main generator output when wheel-slippage is detected on any axle. Reducing the output of the main generator results in decreasing the torque of each of the traction motors. Thus, the output of the main generator can be reduced until the wheel-slippage stops. Although this technique stops the wheel-slippage, it also reduces the amount of power applied to accelerate the locomotive. The torque of each of the traction motors is reduced even though only one of the traction motors may be associated with wheel-slippage. Thus, the maximum amount of power that can be delivered to all of the wheels of the locomotive is limited as a function of the least adhesive wheels.
Another technique used to control wheel-slippage involves applying an abrader, such as sand, between the wheels of the locomotive and the rail to increase adhesion. The application of the abrader inhibits wheel-slippage from occurring. But, once wheel-slippage has started, the abrader is ineffective to stop the wheel-slippage.
Another technique used to control wheel-slippage involves the application of friction brakes on the wheels that are slipping. The friction brakes can be used to slow the wheels to the rack speed of the train, and hence, to stop the wheels from slipping. A problem with this technique exists because typical friction brakes are not designed to operate on one axle independently of the others. Thus, to stop wheel-slippage, the friction brakes are applied to all of the wheels of the locomotive thereby reducing the acceleration power. In addition, typical friction brake controls are not very accurate. Thus, applying the friction brakes can result in one of two extreme conditions. First, if the friction brakes are not strong enough, the wheels may continue to slip after the application of the friction breaks. Secondly, if the friction brakes are too strong, the wheels may begin to slide on the rail.
Another technique used to control wheel-slippage is based on the configuration of the railroad locomotive. Generally, a locomotive or locomotive consist, includes several locomotive units located at the front of a line of rail cars. The locomotive unit located at the very front is more susceptible to wheel-slippage than the other locomotive units. Thus, when wheel-slippage is detected, the wheel-slippage can be cured by reducing the power of only the lead locomotive unit. This method of reducing wheel-slippage is insufficient for at least two reasons. First, when the locomotive only consists of a single locomotive unit, this method is inapplicable. Second, this method is crude and does not take into account other factors such as the wear of the wheels of other locomotive units, output power of the other locomotive units, etc.